Although it can be applied to any desired connecting structures, the present invention and the problems on which it is based are explained in more detail with reference to a fuselage shell of an aircraft stiffened by means of stringers.
Fuselage shells and stringers are increasingly being produced from fiber composite materials, in particular from carbon fiber reinforced plastic (CRP). Fuselage shells and stringers have in this case a construction comprising a number of layers of laid fiber fabric which are joined to one another by means of a cured resin matrix, in particular epoxy resin. Typically used as stringers for stiffening the fuselage shell are T stringers, which have a foot, at which they are connected to the fuselage shell, and a web portion, which extends substantially perpendicularly in relation to the fuselage shell. In this case, the foot of the T stringer is connected to the fuselage shell by means of the resin matrix.
Under normal operating conditions of an aircraft, tensile, compressive and shearing stresses act in the fuselage shell parallel to the plane of the fuselage shell. In addition, a major portion of the fuselage shell is subjected to forces acting perpendicularly in relation to the plane of the fuselage shell, which are produced by the pressure difference between the cabin and the surroundings. In addition, the fuselage shell is occasionally exposed to impact, that is to say brief, intensive force effects. An example of such impact is a piece of rubber of a tyre torn to pieces on landing, which hits the fuselage shell at high speed.
Such instances of impact in such fuselage shells of fiber composite material are often the initial cause of damage in the fuselage shell. They involve a flux of force starting from the impact and extending along the fuselage shell via the foot of the T stringer into the web portion of the T stringer. In particular in the lateral connecting regions of the end of the T stringer foot and the foot region of the T stringer, which lies directly underneath the web portion of the T stringer, very great stress peaks briefly occur in the resin matrix. The consequence is then a brittle fracture of the resin matrix in these connecting regions. Then the T stringer becomes detached from the fuselage shell and a catastrophic collapse of the latter occurs.